This is a competing renewal of a postdoctoral training program entitled "Training in the Pathogenesis of Infectious Diseases." The program based in the Infectious Diseases Section of the Department of Internal Medicine at the Yale University School of Medicine. The program's overall goal is to produce outstanding physician-scientists who use modern laboratory research methods to study human infectious diseases. The program is designed primarily for M.D.'s and M.D./Ph.D.'s who have completed their subspecialty clinical training in the Infectious Diseases fellowship program at Yale, but selected M.D.'s or M.D./Ph.D.'s with clinical training in other fields will also be eligible to participate. The training faculty consists of (i) 16 laboratory-based researchers from two clinical (Medicine and Pediatrics) and three basic science (Microbial Pathogenesis, Cell Biology and Immunobiology) Departments or Sections and (ii) 3 experts in clinical epidemiology and patient-oriented research from the Departments of Medicine and Pediatrics. Each trainee will be supervised by two co-mentors, and their progress will be monitored by Training Committees. The trainees will have access to several new programs and facilities at Yale, including the recently-established Section of Microbial Pathogenesis, an innovative new Investigative Medicine Ph.D. Program for M.D.'s who have completed their clinical training, and a state-of-the-art laboratory research building that has just been completed on the Yale Medical School campus. We are confident that this Program will provide its trainees with outstanding opportunities to pursue careers in infectious diseases research.